hdparm and disk read speed

Question

My hdparm -t buffered disk reads output is around 120 MB/sec. However, I was writing a little code that involves reading a big file (around 400MB), when I managed to read it all in about one second ( fread() on c ). Does the hdparm output not quite say what I understand? Why am I able to read so much faster?

I do very simple processing to the bytes I read, counting how many are the letter 'A'.

Answer 1

hdparm is bypassing your OS's cache, but your test program is taking advantage of it. Presuming you're running Linux, evict everything from the page cache with this command (source: http://linux-mm.org/Drop_Caches ):

echo 3 > /proc/sys/vm/drop_caches

Then, run your test program. It should be at or below the rate reported by hdparm.